Variability in Nest Survival Rates and Implications
to Nesting Studies

Appendix

Method Used to Calculate Mortality Rates by Age Class and Calendar Period

We calculated daily mortality rates for nests in each 5-day period of age (1-5
days, 6-10 days, etc.) and for each 10-day calendar period (Days 110-119, 120-129,
etc.). To do so, we calculated the exposure of all nests in each category and
the number of such nests destroyed. The daily mortality rate is the number destroyed
divided by the number of nest-days exposure. As an example, consider a nest
found at age 3 days (laying with 3 eggs) on Day 118 (28 April). It was checked
on Day 125 (5 May) and found to be still viable. Thus, there were 0 destructions
occurring during 7 nest-days of exposure. The exposure was distributed over
the AGE-DATE categories as follows:

AGE:

3

4

5

6

7

8

9

DATE:

118

119

120

121

122

123

124

Exposure:

1

1

1

1

1

1

1

Losses:

0

0

0

0

0

0

0

Hence the following AGE-DATE categories would receive the indicated contributions
to exposure:

AGE

DATE

1-5

6-10

110-119

2

0

120-129

1

4

Had the nest been destroyed by the visit on Day 125, then one loss would be
attributed to the nest and the exposure reduced, because the expectation of
the number of days the nest was at risk is less than 7. The single loss would
be distributed over the 7 days in the interval according to the probability
that the nest was destroyed on that day. To evaluate that probability, we assumed
that nest mortality occurred at a constant rate of 5%/day. Then the probability
that a nest was destroyed, for example, by the next day (Day 119), if it was
destroyed when next visited on Day 125, is

Pr{ survive exactly 0 days | not survive 7 days}

= Pr{survive exactly 0 days}
1 - Pr{survive at least 7 days}

= 0.1657.

The distribution of the single loss over the 7-day period between visits to
the nest is as shown in the following table:

AGE:

3

4

5

6

7

8

9

DATE:

118

119

120

121

122

123

124

Exposure:

1.0000

0.8343

0.6768

0.5272

0.3851

0.2501

0.1218

Losses:

0.1657

0.1575

0.1496

0.1421

0.1350

0.1282

0.1218

The exposure is also distributed over the interval. The exposure for Day 118
will be 1, because the nest was exposed to risk on that day. The exposure accruing
to Day 119 will be

E{exposure on Day 119 | not survive to Day 125}

= 1 × Pr{survive at least 1 day | not
survive 7 days}

=Pr{survive
exactly j days | not survive 7 days}

=

=

= 0.8343.

The distribution of exposure over the 7 days between visits is as shown in
the preceding table. From that table we can determine the contribution of the
single nest to the losses and exposure for AGE-DATE categories. The following
table shows losses/exposure for each category.

AGE

DATE

1-5

6-10

110-119

0.3232/1.8343

0/0

120-129

0.1496/0.6768

0.5271/1.2842

After adding nest losses and exposure in each category, daily mortality was
calculated by the Mayfield Method.